European banks remain vulnerable to another financial crisis. Capital buffers are too small, creditor bail-ins too low and emergency resolution mechanisms still inadequate to insulate governments from losses that could arise from a system-wide failure. In short, we need a new formula for financial stability.

A banking crisis today would still cost European sovereigns between 2% and 10% of GDP. The final bill would depend on many factors, the size of the initial loss being only one. Others include how much is mitigated by the banks' own capital reserves, by pre-existing government backstops and by any money that can be recouped by bailing-in bondholders.

The size of a country's banking system is also critical. By that measure, Europe's banks remain too big to fail.

Euro-zone lenders have shrunk their balance sheets by a total of €4.4 trillion since the second quarter of 2012, but still hold assets worth more than €30 trillion, according to the European Central Bank. That's three times the output of the euro area and far outstrips the U.S., where bank assets are less than GDP.

And if European banks are too big, they're also still undercapitalized. Capital is adequate relative to risk-weighted assets, against which "capital ratios" are measured, but falls short as a proportion of the banks' total asset base and of potential losses.

To make their capital ratios look better, many banks have reduced their risk-weighted assets over the past few years, often by "optimizing" their internal risk models. More than one-third of European banks now have less than 30% risk-weighted assets over total (RWA), some as low as 20%. This means a bank's "10% capital ratio" is effectively equal to €2 of capital for every €100 of assets (20% RWA times 10% equals €2). That's too low, especially for systemic banks whose balance sheets are as large as a country's GDP.

Regulators recognize the issue and are trying to introduce an absolute floor on capital: the Basel committee's 3% leverage ratio prescribes a minimum €3 of capital over assets. But even this would not have helped troubled banks such as Dexia or Anglo Irish, which lost the equivalent of 4% and 20% of their assets during the 2008-09 crisis, respectively.

We estimate that to stand on their own feet, banks need a leverage ratio of about 5.8% of capital over assets. This is consistent with the approach of Swiss and U.S. regulators, who recommend a 6% leverage ratio. It means euro-zone banks would need to raise an additional €492 billion of capital—more than six times the €80 billion that the European Banking Authority says they raised in 2013.

Another solution would be to increase bail-in requirements or state backstops. German Finance Minister Wolfgang Schäuble recently proposed speeding up the formation of Europe's Single Resolution Fund, a bank-financed pool of money planned to help wrap up or restructure failing banks. But the proposed €55 billion that would be available in the fund pales in comparison to the potential losses generated by bank failures, even if a bail-in were to be implemented first. We estimate the fund could help one large or two mid-sized institutions withstand failure, at best.

These backstops are also very difficult to put into action—the decision to restructure or resolve a bank has to pass through national committees, the European Commission, the Single-Resolution Mechanism (whose fine-print is still being drafted), and various other boards. Not a weekend job.

Regulators need a more comprehensive approach to making banks safe. It must encompass the total size of capital reserves, the size and structure of banking systems, and rules that can efficiently bail-in bondholders. Regulators are moving in the right direction, but they have not gone far enough.

Ultimately, I believe that Europe will be free from the threat of failing banks only once it has a smaller banking system and a more diversified supply of credit.

Think of credit like an energy grid: In Europe, 80% to 90% of the energy comes from banks—the coal or nuclear plants of the system. If something goes wrong with them, the costs will be high, the collateral effects toxic, and the damage could take years to clean up.

We need more "renewable energy" in the form of non-bank sources of credit. That means bonds, securitizations, and lending from insurance companies and asset managers. Only then will Europe be free from its banks.Mr. Gallo is the head of macro-credit research at the Royal Bank of Scotland. The views expressed are his own.